The spread of personal computers and workstations has led to the development of networks for interconnecting such equipment and common resources such as printers and data storage devices. More powerful and sophisticated computing equipment and programs have progressively become available, allowing the processing of data in larger and larger quantities, for example in the form of database information and graphic images. These developments have in turn placed increasing demands on the speed and capacity of networks.
Various new networking technologies have been proposed to cater for these demands. One such technology is the fibre distributed data interface (FDDI), which is based on the use of optical fibres and optical signals. However, practical experience has shown that although this technology can provide the required performance, it is relatively expensive, requiring the costly process of installing complete new networks of optical fibre, which is itself inherently expensive.
Accordingly attention has also been directed to the possibility of transferring data at high speed over existing wiring installations, thereby avoiding the cost of installing a new network and gaining additional return for the expense previously incurred in installing the existing wiring.
One possible technique along these lines involves the use of unshielded twisted-pair (UTP) telephone cables of the kind already used for lower-speed local-area networks. In this technique, described for example in U.S. Pat. No. 5,119,402, the required high data rate is achieved by transferring the data over multiple conductors, so that different portions of the data are transmitted simultaneously over respective conductors.
Although this technique provides many advantages, it also suffers from potential specific problems. In particular, a primary source of noise affecting such transmissions is electrical impulses, which can affect all conductors together for an extended period relative to the duration of transmission of a data bit. Thus, for example, twenty successive bits of the data to be transmitted may be split into four blocks of five bits each, and encoded by selecting six-bit code values in accordance with the five-bit values from a predefined 5B/6B code table. The four encoded blocks are then transmitted as a group simultaneously along respective ones of four conductors in a multi-conductor cable. It is possible for two successive groups of four blocks each to be affected by electrical noise. Since one group of blocks represents twenty bits, a total of forty bits may be corrupted. Although the data may be protected against some corruption by the use, for example, of a cyclic redundancy check (CRC) code, such codes are typically based on polynomials of degree thirty-two at most to limit the associated processing burden. Such a CRC code can be guaranteed to detect the corruption of at most thirty-two consecutive bits in the data stream, so corruption of forty successive bits can lead to errors which the CRC code does not detect.
It is possible to improve the resistance of such a communication technique to noise-induced errors by selection of particular data word/code word combinations. However, the identification of such combinations has up to now been a difficult and time-consuming procedure.
It is an object of this invention to provide a method and coder for encoding data, for example for communication using cables with multiple conductors, which at least alleviates these problems.